Q. Why does PV, plasma volume, rise in polycythemia vera? This seems counterintuitive. What factor(s) are responsible for increasing plasma volume in this condition? I’m guessing it is some sort of compensatory response by the liver (plasma proteins/osmotic balance) or the kidneys (fluid volume management) to the huge rise in red cell numbers.
A. The plasma volume rises in PV for two reasons:
1. Hypersplenism (patients with a large spleen experience an increase in plasma volume, due – at least in part – to the pooling of red cells in the spleen, and a compensatory increase in plasma volume in the rest of the circulation). Most patients with PV have at least some degree of hypersplenism.
2. A gradual compensatory increase in vascular capacity. The mechanism for this is not well understood – it’s more of a theory.
In polycythemia vera, the increase in red cell mass is a more gradual process than it is in secondary polycythemia (such as that due to hypoxia). This probably explains why you see an increase in plasma volume in primary, but not secondary, polycythemia.
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